Method of burning magnesium and calcium carbonate materials



Dec. 9, 1969 E. EINSTEIN ETAL METHOD OF BURNING MAGNESIUM AND CALCIUM CARBONATE MATERIALS Filed April 19, 1968 RAW CHARGE MATERML HEAT EXCHANGE Exhaust 4nd Freed C0 I A C ombus a' on Pradacs And l-7-eed Cambaston Producs T/flnd Freed 50 Charge HEAT EXCHANGE PRODUCT EDWARD EINSTEIN 4?.

DAVID J. N ELL 5) United States Patent O US. Cl. 26353 3 Claims ABSTRACT OF THE DISCLOSURE A method of burning materials containing calcium carbonate and one or more sulfates in recuperative-type furnaces whereby substantially all the sulfate is removed from the materials.

BACKGROUND When one or more sulfates, for example, calcium sulftaes or magnesium sulfate is or are present in a mixture of magnesium carbonate or calcium carbonate materials, it is often desirable to substantially eliminate the sulfate when burning to remove the CO For example, limestone burnt for use in preparing steelmaking slags, especially in the basic oxygen process, must contain less than about 0.04% S Sulfate in excess of this amount tends to reduce the activity of the burnt lime, thereby slowing the rate at which it enters the slag. In the manufacture of magnesia from sea Water or brines, calcined dolomite is mixed with sea Water to precipitate the magnesium values. If the sulfur content of the dolomite is excessive, the dolomite is very slow to enter the reaction and is referred to by those in the industry as killed.

For burning processes to be commercial, it is necessary that they be both continuous and recuperative. By recuperative is meant that the exhaust gases escaping from the kiln be used to preheat the incoming charge material and that the exiting charge material be used to preheat the incoming combustion air. Recuperative-type furnaces include shaft kilns, rotary kilns, and multiple hearth kilns, among others.

Sulfates can be removed from materials containing calcium carbonate and magnesium carbonate by expelling the sulfur in the form of gases. The addition of coke or other reducing agents to the mixture prior to burning facilitates expulsion of sulfur in the gaseous form. The addition of the reducing agent causes reduction of the sulfates which, through a series of reactions, is ultimately converted (in the case of calcium sulfate) into calcium oxide and sulfur dioxide. Typical chemical reactions are:

CaSO +CO=CaO+SO +CO 1 CaSO +4CO=CaS+4CO 2) 3caso,+cas=4so +4cao 3 The amount of the reducing agent used to reduce the sulfate must be carefully controlled since an excess will cause a further reduction, wherein part of the sulfate will be reduced to sulfide (reaction 2). If sulfate remains, the sulfide will react with it expelling S0 (reaction 3). If no sulfate remains, the sulfide can be decomposed only by excessive heat (2550 F.) creating a less active product and, therefore, will be retained in the final product.

As the reactions set forth above are reversible, removal of sulfate from calcium carbonate-containing materials cannot be performed in recuperative-type furnaces of the type described above. The sulfate is removed at temperatures above about 2000 F. when the atmospheres are slightly reducing. The released sulfate becomes part of the exhaust gases. As the exhaust gases move through the 3,483,280 Patented Dec. 9, 1969 incoming charges to transfer their heat thereto, the sulfates are redeposited upon the incoming material. This is especially true if the exhaust gases passing through the incoming material are not maintained slightly reducing.

It is an object of this invention to provide a continuous method of burning materials containing calcium carbonate and one or more sulfates in recuperative-type furnaces, wherein the sulfate is substantially entirely removed. It is another object of this invention to provide a method of burning dolomitic materials containing up to 5% S0 wherein the product is suitable for use in the manufacture of magnesia from sea water or brines. It is yet another object of this invention to provide a method of burning limestone and similar materials containing up to 5% S0 whereby the product is suitable for the preparation of slags in basic oxygen steelmaking processes.

BRIEF DESCRIPTION OF THE INVENTION According to this invention, a method of burning materials containing calcium carbonate and one or more sulfates, for example, limestone or lime sands or dolomite, are burnt in a continuous process in a recuperative-type furnace. In this process, the materials are heated in two stages. The charge materials are heated in an oxidizing or neutral atmosphere to about 1800 F. by the combustion of fuels and air. During this stage, substantially all the CO is released from the carbonates. The combustion products and freed CO produced in this first stage are used to preheat the materials prior to entering the first stage. In the second stage, the charge materials are heated in a reducing atmosphere by the combustion of fuels With insufficient air to a temperature of about 2000 to 2400 F. During this reducing stage, the remaining CO is freed from the carbonates and the S0 is removed from the sulfates. A portion of the combustion products and freed CO and S0 produced in the reducing stage are then passed through the charge materials being treated in the first or oxidizing stage. The remaining portion of the combustion products and freed CO and S0 produced in the reducing stage, are exhausted. The ratio of the portion passed into the oxidizing stage and the portion eX-, hausted is adjusted such that substantially all sulfur is removed from the material.

DETAILED DESCRIPTION Further features and other objects and advantages of this invention will become clear to those skilled in the art by a careful study of the following detailed description. In this specification and appended claims, all percentages and ratios are by weight; chemical analyses were obtained by spectrographic analysis with control by wet chemical analysis, and are reported as oxides.

This sole drawing is a flow diagram of the process according to the teachings of this invention. In the diagram, the charge material moves from top to bottom as it would in a shaft kiln or a multiple hearth kiln. The charge first enters a heat exchanger and then passes into an oxidizing or neutral burn wherein most of the carbonate is removed. Thereafter, the charge passes into a reducing burn wherein the remaining CO is removed and the S0 is removed. The charge finally passes into an optional heat exchanger and is recovered. Air for introduction into the reducing or the neutral to oxidizing burn may be preheated in the lower heat exchange step. The oxidizing burn is heated by combustion of fuels and air. The reducing burn is heated by combustion of fuels with insufficient air to completely oxidize all of the fuels. The combustion productsand CO formed in the neutral or oxidizing burning stage are passed into the upper heat exchange, and thereafter exhausted. The combustion products and freed S0 created sasazso in the reducing burn are handled in a very special manner. A portion of these products are passed into the oxidizing or neutral burn to recover unburned fuel values and heat. Another portion is exhausted. The ratio of the portion passed into the oxidizing to neutral burn and that porhearth furnace. The S was removed on the reducin hearth in the rotary hearth furnace. As a portion of the products of combustion from the reducing hearth were passed into the Herreshoff furnace, the S0 content on the number 2, 3, and 4 hearths was increased. This is o tion exhausted is adjusted so that substantially all the sule the S03 reatlaches self to fur is removed from the product. If all of the products of by cxhaustmg r f one'half F t the reducing burn were passed into the oxidizing to neu- Si qg rotary f g i h P3 2 tral burn, it would not be possible to remove $0 from the i g e f g i i f g i si Product. The S02 freed in the reducing burn would com- 0 i i g g gz m e p a UC on o S y e bine'with the material in the oxidizing to neutral or in l l b h ti 1 the upper heat exchanger, and thereby be returned to the Sands Priman y on the num e1 3 earthvof the m reducing burn. Eventually, the material entering the re- EXAMPLE II dhclflg h wouldflonlaill e Sulfur than h? 5 The apparatus described above was used for treating a moved III the h and It would p 011 Wlth the high sulfur dolomite material. The significant process data product. The optlmurn ratio of the portion of the products are given i bl 11 TABLE II Rotary hearth Multiple hearth furnace furnace reducing lead 1 2 4 (1.5 to 1.7% material Oxidizing burn combustibles) Product Temporal; F 9 so. eei dii basis) ,percel1t 1.09 -fh?1. -fi??fi??9. "$353733 'ilih iiii jiliilifl? "bibi bf Loss on ignition, percent 27. 5 0, 03-0, 0, 07-1. 03 0. 08-0. 32

of the reducing burn to p d n the oxidizing bum Table II establishes that dolomitic materials can be sucand those to be exhausted 1S determinable once a furnace cessfuny proiwssed according to the teachings f this has been installed and a process has been started up. It vemion, In both Examples 1 and 11, 30 was reduced to depends to a large extent on the temperatures used m 1 h ()5% the oxldlhlng and neutral burns and the efiechvenes of Commercial processes reportedly require sufiicient fuel the u s burn for al f u a s. e o of to provide 4x10 and s 10 B.t.u.s/t0n of limestone h p n exhausted, to the pertlon passed Into the calcined. if a one-stage process is used in which the total dlzmg of neutral hum Should P Y hehetween 9 2 3r furnace is maintained reducing, and all gases are dis: a 20180 The larger the w p d o e oxldlzcharged at 2200 F., the fuel requirement would be that mg or neutral burn, the greater the heat saving. On the which would provide between 40 1 e and 50 1Q6 other hand, dependmg on amount of 3 1n the Charge B.t.u.s/ ton. Applicants process, however, which has a material, the larger the portion exhausted the less SO 1n reducing Step requires between x106 and 0 10 the product. It is within the concepts of this invention 40 gings/ton, to exhaust all the Comhhshon Products of the Teduclhg Having thus described the invention in detail, and with u Into the atmospheresufiicient particularity as to enable those skilled in the art to practice it, what is desired to have protected by Letters EXAMPLEI Pt t tf th' thfll l' aenisse or i eoowln calms: The process, accordmg to the teachings of this mven- We claim: n g was Practlced on hme sands by the use of two 1. A method of burning materials containing calcium naces' one fiumace was a a m Herreshofi'type carbonate and one or more sulfates comprising the steps nace 1n which the charge material is fed on the upperof: rglostthhealiithihThedcharge nilaterlal II110VS Iacross c itgh (1) preheating thematerials ear an en mp8 to t 6 next ower earth 6 (2) heating the preheated materials in an oxidizing to gi are i from g gi and neutral atmosphere to about 1800 F. by the introe c arge ma erla 15 remove rom e owest or num her 4 hearth and passed into a separate furnace This ductlon of combustible uels ar-ld an to free from the carbonates, withdrawing the combustion furnace was a single hearth rotaryhearth furnace. Burners roducts nd freed CO and assin them thmu h were placed in the number 3 and 4 hearth of the Herrep t l b h t t g g shoif furnace and in the single hearth furnace. 1 1 8 pre j e m 8 ep d The following table gives the temperature and selected (3) eatmg t 6 {natural 9 step 2 m a ucmg atmoschemical analyses of the charge material and the material Phere h mtroducnon of combusctlblfi fuels and on each hearth and the temperature of each hearth in mshfhcleht to from 2000 to 2400 to free the process according to this invention. mammg 2 m the carboljlates and S02 from Table I establishes that lime sands comprising as much sulfates, Withdrawing a F {1011 Of the Comhllshon as 0.6% S0 (on a calcined basis) can be burnt, acproducts and freed 'CQQ andSO andpassing them cording to this invention, with removal of all but 0.03 directly through the materials being treated in step TABLE I i Rotary hearth Multiple hearth furnace furnace reducing Charge 2... 3 4 (1.0to2.2% material oxidizing or neutral combustibles) Product.-

Temperature, "F 730-1, 020 1,130-1, 435 1,430-1, 720 1, 790-1, 860 2, 040-2, 220 S03 (calcined basis),percent 0.6 0. 6-0. 85 0. 61-1. 20 0.7 72.52 .s. 0. 03-0. 04 7 Loss on ignition, percent 44 p 3. 2 0.1 Trace 1 Presence of combustibles measured by Bailey Meter evidences reducing atmosphere.

to 0.04%;SO Table I establishes by the change in ignition loss further that the CO is removed from the lime and exhausting the remaining portion, the ratio of the portion passed into step 1 to the portion ex- 6 hausted adjusted such that substantially all sulfur is combustion products of step 3 are exhausted into the atremoved from the material, and mosphere. (4) recovering the burnt material substantially free of References Cited sulfur. 2. Method according to claim 1 in which the ratio of 5 UNITED STATES PATENTS (a) combustion products and freed CO and S0 pro- 2,945,688 7/1960 Pajenkamp et al. 26353 duced in step 3 and passed onto step 2 to (b) the portion 3,044,756 7/1962 Grebe et a1. 26353 of the combustion products; and freed CO and S0 exhausted to the atmosphere is between 20:80 and 80:20.

3. The method according to claim 1 in which all the 10 JOHN I CAMBY, Primary Examiner 

